1. Field of the Invention
The present invention relates to an undesired signal canceller in the vestigial sideband modulation system used in a television receiver and the like, and particularly is to design for cancelling undesired signals at a video amplifier stage after detection by using a widely-used transversal filter (electric charge transfer type programmable filter).
2. Description of the Prior Art
For better understanding, a description will first be given on transfer function representation of an undesired signal in the case that a signal of vestigial sideband modulation system is synchronously detected.
A received signal P(t) including an undesired signal is considered as a mixture of desired signal D(t) and undesired signal U(t), which is expressed as follows: EQU P(t)=D(t)+U(t) (1)
The desired signal D(t) is a linear addition of a 0.degree. or 180.degree. out-of-phase component (hereinafter referred to as an in-phase component) g(t) and a 90.degree. or 270.degree. out-of-phase component (hereinafter referred to as a quadrature component) g.sub..beta. (t), which is expressed as follows: EQU D(t)=1/2g(t) cos .omega..sub.c t+1/2g.sub..beta. (t) sin .omega..sub.c t (2)
where g(t) is a video signal and g.sub..beta. (t) is expressed as follows: ##EQU1## Also, W.sub..beta. (.omega.) is a vestigial sideband roll-off characteristic as shown in FIG. 1 and represented as follows: ##EQU2##
Meanwhile, an undesired wave (a single wave) passed through (i)th propagation path is expressed as follows: EQU U.sub.i (t)=R.sub.i D(t-.tau..sub.i) (6)
where R.sub.i is an amplitude ratio for the desired signal D(t) in a relation of 0&lt;R.sub.i &lt;1 when i is not zero and R.sub.i =1 when i is zero, and .tau..sub.i is a delay time (sec) which is greater than zero when i is not zero and zero when i is zero.
When the equation (2) is substituted into the equation (6), the following relation is obtained. ##EQU3## If a carrier phase .psi..sub.i (rad) is taken as follows: ##EQU4## the following relation is obtained ##EQU5##
In general, when n's number of undesired waves are present, the undesired signal U(t) is expressed as follows: ##EQU6## Thus, the received signal P(t) is expressed as follows: ##EQU7##
If the received signal P(t) is synchronous-detected at a phase difference of zero with the carrier phase of the desired signal D(t), the following video signal p.sub.0 (t) is obtained. ##EQU8## If the equation (11) is subjected to Fourier transformation in consideration of the equation (3), the following equation is obtained. ##EQU9##
In the above equations (12) and (13), H.sub.g0 (.omega.) is a transfer function of the undesired signal U(t) which is synchronous-detected at the video amplifier stage. Further, with the equation (13) being subjected to Fourier inverse-transformation, the impulse response of undesired signal U(t) at the propagation path is expressed as follows: ##EQU10## and a mark * indicates convolution.
Now, the following relation is obtained from the equation (12): ##EQU11## Therefore, if P.sub.0 (.omega.) is multiplied by an inverse filter H.sub.r0 (.omega.) of the transmission system expressed as follows: ##EQU12## the desired signal G(.omega.) only can be derived from the received signal P.sub.0 (.omega.). The inverse filter H.sub.r0 (.omega.) as expressed in the equation (16) can be realized by a construction called a feed-back system as shown in FIG. 2.
As described above, if it is possible to form a circuit simulating the transfer function of undesired signal U(t) or its impulse response, the undesired signal U(t) can be cancelled by forming an inverse filter which includes the above circuit in its feed-back loop.
As the above-mentioned circuit simulating the transfer function of undesired signal U(t) or its impulse response, there has been proposed a circuit as shown in FIG. 3.
In FIG. 3, A.sub.1, A.sub.2 . . . A.sub.n represent delay circuits respectively corresponding to delay times .tau..sub.1, .tau..sub.2, . . . .tau..sub.n of undesired waves arrived passing through different propagation paths. These delay circuits A.sub.1 to A.sub.n are connected in series and also have delay amounts of ##EQU13## respectively.
Outputs of the delay circuits A.sub.1 to A.sub.n are respectively fed to phase and level adjusting circuits B.sub.1 to B.sub.n and C.sub.1 to C.sub.n. In accordance with levels R.sub.1 to R.sub.n of undesired waves and phases .psi..sub.1 to .psi..sub.n thereof, the adjusting circuits B.sub.1 to B.sub.n respectively produce signals which are adjusted to R.sub.1 cos .psi..sub.1, R.sub.2 cos .psi..sub.2, . . . , R.sub.n cos .psi..sub.n, while the adjusting circuits C.sub.1 to C.sub.n respectively produce signals which are adjusted to R.sub.1 sin .psi..sub.1, R.sub.2 sin .psi..sub.2, . . . , R.sub.n sin .psi..sub.n.
Thus adjusted signals from the circuits B.sub.1 to B.sub.n and C.sub.1 to C.sub.n are respectively supplied to adders 1a and 1b. In the adder 1a, with respect to undesired wave as shown in FIG. 4A, a pulse signal proportional to the level of an in-phase component of the undesired wave is formed .sup..tau. D/2 before the generation of the undesired wave as shown in FIG. 4B. Also, in the adder 1b, a pulse signal proportional to the level of a quadrature component of undesired wave is formed .sup..tau. D/2 before the generation of the undesired wave as shown in FIG. 4C.
A signal from the adder 1a, which is proportional to the level of in-phase component is supplied to a delay circuit 2a having a delay time of .sup..tau. D/2 to form a signal as shown in FIG. 4D, which is equivalent to the impulse response of the in-phase component of undesired wave.
A signal from the adder 1b, which is proportional to the level of quadrature component, is supplied to a filter 2b having characteristics corresponding to the impulse response of vestigial sideband roll-off characteristic as shown in FIG. 5.
The filter 2b is formed with a transversal filter as shown in FIG. 6. In other words, delay circuits D.sub.1 to D.sub.m corresponding to each sampling period T are connected in series, and outputs of these delay circuits D.sub.1 to D.sub.m are respectively fed through level adjusting circuits E.sub.1 to E.sub.l to adders 4. The level adjusting circuits E.sub.1 to E.sub.l are given with sampled values of impulse response shown in FIG. 5. The sampling frequency f=1/T is selected to be more than two times the video signal frequency band and, for example, three times or four times the color subcarrier frequency.
Then, the filter 2b has derived therefrom a signal which is equivalent to the impulse response of quadrature component of undesired wave as shown in FIG. 4E. This signal from the filter 2b is applied to an adder 3 where it is added to the signal from the delay circuit 2a thereby to form a signal which is equal to the undesired wave (FIG. 4A).
Accordingly, if the above circuit of FIG. 3 is provided in the feed-back loop of FIG. 2, the undesired signal can be well removed. This effect was also noticed from computer simulation and circuit experiments.
In this method, however, the undesired signal is divided into an in-phase component and a quadrature component so as to be cancelled therefrom. As a result, it is required to separate these in-phase component and quadrature component for detection so that the above procedures become complicated. Thus, this circuit is not suitable for automatic cancellation of undesired signals.
As a method of automatically detecting the undesired signal U(t), there is used a vertical synchronizing signal in a standard television signal. This signal is defined, for example, as shown in FIG. 7 by Japanese Radio Law, in which no signal appears during an interval of 29.3 micro seconds (.mu.s) from an equalizing pulse and an interval of 27.3 .mu.s up to the vertical notch. Besides, the vertical synchronizing signal is of a step responsive waveform. Therefore, when an undesired signal exists, the flat portion of the vertical synchronizing signal is distorted and this distortion will represent response characteristics of a transmission path of a television signal. Accordingly, parameters of the filter H.sub.r0 (.omega.) may be so established that this distortion is less than the permitted limit.
In this case, it is considered that an undesired wave of in-phase component as shown in FIG. 8C is produced as a result that an impulse of in-phase component having a level of -0.5 which is delayed from the reference signal (vertical synchronizing signal) by .tau..sub.1 =kT as shown in FIG. 8B is convolution-integrated with the reference signal shown in FIG. 8A.
Accordingly, there has been proposed a device using a transversal filter as shown in FIG. 9. In this circuit, a video signal including an undesired wave is supplied from an input terminal 5 through a subtractor 6 to an output terminal 7. Delay circuits F.sub.1 to F.sub.k corresponding to sampling period T are connected in series and the output signal from the subtracter 6 is applied to the delay circuit F.sub.1. Outputs of the delay circuits F.sub.1 to F.sub.k are respectively fed through level adjusting circuits G.sub.1 to G.sub.k to an adder 8 the output of which is supplied to the subtracter 6. Further, the output signal of the subtracter 6 is also supplied to an undesired signal detecting circuit 9. An output of this detecting circuit 9 is applied to a gain control circuit 10 which controls gains of the level adjusting circuits G.sub.1 to G.sub.k.
With the circuit of FIG. 9, a given reference level .epsilon. is established at the detecting circuit 9. When an undesired signal exists at the flat portion of the vertical synchronizing signal, a time point at which the level of the undesired signal first exceeds the level .epsilon. is detected on the time basis so that one of the level adjusting circuits G.sub.1 to G.sub.k corresponding to the above time point is controlled in gain to produce a predetermined impulse. Thus, the gains of level adjusting circuits will be adjusted until the level of the undesired signal is lowered to .epsilon. or less.
With the arrangement as mentioned above, the impulse as shown in FIG. 8B is convolution-integrated with the reference signal by the transversal filter to form the signal as shown in FIG. 8C and this signal is subtracted from the reference signal shown in FIG. 8A so that the undesired signal can be cancelled.
In the circuit of FIG. 9, however, the impulse is convolution-integrated as mentioned above and hence a signal fed to the subtracter 6 will have a stepped waveform. For this reason, there is no problem when the undesired signal is only in-phase components as shown in FIG. 8C, but when the undesired signal is quadrature component approximated to a differential waveform, the undesired signal can not be cancelled, and the component of undesired signal will contrarily become large.
For example, with respect to a quadrature component as shown in FIG. 10A (numerals indicate levels with that of the reference signal being taken as 1.000), if a suitable impulse is formed at a point where the undesired signal is first risen above the reference level .epsilon., this impulse is convolution-integrated to form a signal as shown in FIG. 10B which is supplied to the subtracter 6. In FIG. 10B, an arrow denotes the impulse. The subtracter 6 then produces a signal as shown in FIG. 10C. These figures such as FIG. 10B, 10C, etc. show only a portion of undesired signal.
With respect to the signal shown in FIG. 10C, a second impulse is formed at a point where the undesired signal is first risen above the level .epsilon.. Thus, the subtracter 6 is applied with a signal as shown in FIG. 10B' to produce a signal as shown in FIG. 10C'.
Furthermore, with respect to the signal shown in FIG. 10C', when a third impulse is similarly formed, the subtracter 6 is supplied with a signal as shown in FIG. 10B" to produce a signal as shown in FIG. 10C". As illustrated, the level of undesired signal at a time point where the third impulse is formed becomes about three times as large as that of the initial undesired signal, so that if the above process is repeated, the level of undesired signal will be further enhanced. Accordingly, it is not suitable to use this method in the transmission system including an undesired signal of quadrature component for cancelling the undesired signal. On the other hand, with the circuit of FIG. 9, there has been proposed a method of using the detecting circuit 9 to produce a differential waveform of an undesired signal thereby to control the gain of a level adjusting circuit. Since an output of the transversal filter is an integrated waveform of an impulse, if the differential waveform of undesired signal is supplied to the transversal filter, a signal approximated to the undesired signal can be formed.
In this case, however, the waveform of the vertical synchronizing signal is not a perfect step shape but a little gentle in slope as shown in FIG. 11A. Therefore, the undesired signal also has a rising portion with a gentle slope so that such an undesired signal is differentiated to form a signal having a constant width as shown in FIG. 11B. When this differential waveform is convolution-integrated by the transversal filter, a signal having a more gentle rising portion is formed as shown in FIG. 11C. Even if this signal is supplied to the subtracter 6, an undesired signal as shown in FIG. 11D will remain at the output side of the subtracter 6.
As shown in FIGS. 11B' and 11C', it is also possible that the level adjusting circuit to be controlled is shifted forward one by one. In this case, however, there still remains an undesired signal as shown in FIG. 11D'. An effect according to this method is the same for quadrature components.
As described above, the prior art methods have drawbacks such that automatic cancellation of undesired signals is improper, quadrature components thereof can not be eliminated completely, a residual undesired signal appears or the like.